The Dow isn't the only index taking a hit this week. Stock markets around the world have fallen, pushing down the portfolio values of exchange-traded funds. These 16 ETFs had the lowest average price-to-earnings ratios on their portfolios. The prices of the funds' holdings have dropped faster than their reported earnings, suggesting the portfolios might be undervalued. While these ETFs might fall further, low price-to-earnings ratios can reduce the risk of losing.